Project Summary Our long-term goal is to study the ubiquitin system in immune regulation. We initiated a new study of the deubiquitinating enzyme CYLD and found that deficiency of CYLD in T regulatory cells (Treg) caused chronic lung inflammation. Those mutant Tregs expressed altered patterns of chemokine receptors and increased IL-4 production. These preliminary studies pointed to new mechanisms of CYLD regulation of Tregs, via modulating their homing/migration and cytokine production, and they highlight potential tissue-specific aspects of Treg function. Recently, we found that CYLD acts as a positive regulator in the differentiation of T follicular regulatory T cells (Tfr). These preliminary studies thus provide us with a solid basis for testing our central hypothesis that the ubiquitination/deubiquitination system plays a critical role in the immune regulation via modulating different T cell subsets. Here we plan to test this hypothesis by proposing two Specific Aims: Aim 1, to study CYLD in Treg regulation; and Aim 2, to study the role of CYLD in Tfr. The expected results will significantly advance our basic knowledge on the protein deubiquitination pathway in immune regulation, and will provide clues to therapeutic intervention of human diseases.